1. Field of the Invention
The present invention relates to radio frequency (RF) receivers, and more particularly, to RF receivers with tracking filter banks for single conversion broadband filters.
2. Description of the Related Art
Direct-conversion, or homodyne, receivers are popular for many communications applications because of their simplicity and low power. They do not require intermediate-frequency (IF) filters, which are often costly, and need only one frequency conversion stage and one local oscillator (LO).
Direct-conversion receivers rely entirely on quadrature mixing to obtain the necessary image rejection. Single-conversion low-IF receivers for broadband communication systems, such as direct broadcast satellite (DBS), broadcast or cable television (CATV), may rely on some combination of quadrature conversion and pre-selection with tracking or switched filters.
Double-conversion receivers for these applications rely more on IF filtering for image rejection. Because the IF frequencies can be fixed, very sharp filters, such as surface acoustic wave (SAW) filters, need to be used. However, these filters are relatively costly.
Difficulties in integrating tracking filters and in achieving the necessary degree of image rejection through quadrature balance have heretofore prevented the implementation of integrated direct-conversion receivers for broadcast television and CATV. Integrated single-conversion low-IF tuners generally do not include the tracking filters.
A typical conventional single-conversion tuner is illustrated in FIG. 1. As shown in FIG. 1, the conversion tuner includes a bank of bandpass filters 101a, 101b, 101c (in this example, three filters) that receive RF input. The output of the bandpass filters 101 is fed into a low-noise amplifier 104, and then to a mixer 109. The tuner also includes a phase-lock loop (PLL) 120, which is comprised of a phase-lock loop controller 105, and voltage controlled oscillators (VCO's) 106a, 106b, 106c. Each VCO 106 is matched to a corresponding bandpass filter 101. The output of a phase-lock loop controller 105 controls the filters 101 and the voltage control oscillators 106. The outputs of the VCOs 106 are also inputted into the mixer 109.
As further illustrated in FIG. 1, the output of the mixer 109 is received by a variable gain amplifier (VGA) 110, which is placed in a feedback loop that includes a power detector 111. The output of the variable gain amplifier 110 then is inputted to a downstream demodulator (not shown in FIG. 1).
Thus, each tracking filter 101 is slaved to a corresponding voltage-controlled oscillator (VCO) 106a, 106b, 106c, which produces the local oscillator (LO) for the first frequency conversion when that filter 101 is selected. The choice of filter 101 and VCO 106 depends on the frequency of the desired channel, and is determined by a digital command. By using matched components, such as inductors, capacitors, and voltage-variable capacitors (varactors) in the VCO 106 and filter 101, the center frequency of each filter 101 can be matched to the frequency of the corresponding VCO 106. This arrangement is illustrated in FIG. 2.
As shown in FIG. 2, each bandpass filter 101 can include an inductor L205, a capacitor C207, a varactor diode D208, and a capacitor C206, connected as shown. Choke inductors L204 are used to prevent leakage of RF signals back upstream. As further shown in FIG. 2, the voltage control oscillator 106 includes active components 202, and passive components L210, C212, D213, and C211, connected as shown. A choke L209 similarly prevents leakage of RF signals back upstream from the VCO 106.
A secondary problem in direct-conversion tuners are the responses at the LO harmonics, as shown in the bottom graph of FIG. 1. For a narrowband communication system, where the input spectrum spans less than one octave, this is not a problem. There are no signals to be received at the frequencies corresponding to the LO harmonics. Any unwanted signals can in principle be removed with a fixed filter. But for wideband communication systems such as CATV and terrestrial broadcast television, this issue must be addressed. For example, for a TV channel at 50 Mhz, there is an unwanted harmonic response at 150 Mhz, as shown in FIG. 1.